Stabilization of organic isocyanates



STABILIZATION OF ORGANIC ISOCYANATES Louis Spiegler, Woodbury, N.J.,assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., acorporation of Delaware N Drawing. Application January 27, 1956 SerialNo. 561,912

4 Claims. (Cl. 260-453) This invention relates to organic compounds, andmore particularly to the stabilization of organic isocyanates againstdiscoloration.

Organic isocyanates, which in general are colorless liquids or solids,tend to become discolored on storage. On discoloration these organicisocyanates change to from light yellow to brown in extreme cases, andwhile this color can be removed by distilling the isocyanates, thedistilled products again become discolored on standing even in only afew hours.

The discoloration of these organic isocyanates presents a seriousproblem where colorless or very light colored reaction products are tobe produced, for it is usually impractical to redistill the isocyanateimmediately before use.

The exact cause of the color formation in the isocyanates is not known,for it has been found that when these products are stored in the lightor in the dark even under nitrogen, the discoloration developspractically as fast as when the same products are stored in air. Thepresence of moisture is believed to be a factor in causing thisdiscoloration, but, again, it is impractical to package isocyanates,particularly when used in large commercial quantities, under absolutelymoistureproof conditions.

It is therefore an object of the present invention to provide a simpleand positive method for stabilizing organic isocyanates againstdiscoloration. It is a further object of the invention to produceorganic isocyanates which for extended periods of time do not becomediscolored even in the presence of oxygen, moisture or light.

The present invention comprises the stabilization of organic isocyanatesagainst discoloration by incorporating in the organic isocyanate from0.01% to 0.5% of an essentially colorless compound of the groupconsisting of alkyl and aryl esters of carbonic acid, the aryl groups inthe aryl esters being of the benzene series, which is essentiallynon-reactive with organic isocyanates under ordinary atmosphericconditions, or reacts very slowly in the absence of catalysts.

These stabilizing agents are soluble in the organic isocyanates, whichare normally liquids and may be readily dissolved in the melted organicisocyanates which at normal temperatures are solids. While ordinarilythe amount of stabilizing agent employed will vary from 0.01% to 0.5based on the weight of the isocyanate to be stabilized, variousstabilizers hereinafter listed can be used in lesser amounts. Largeramounts of any of the stabilizers of course may be used up to theirlimit of solubility in the isocyanate, but unnecessary large amounts ofcourse are to be avoided since they merely constitute an added impurityin the product. When the very small amounts are used, the elfect of thisimpurity on the product is negligible.

The following list of aliphatic and aromatic alkyl and aryl esters ofcarbonic acid, when tested in the concentrations within the limits ofthose specified above and atent 2,885,423 Patented May 5, 1959 ExampleToluene-2,4-diisocyanate is distilled at 4 to 5 mm. of mercury pressureand the colorless fraction distilling at 106 to 108 C. is collected in afour unit multiple receiver, about 45 g. being collected in each. Thesystem is raised to atmospheric pressure by admitting dry nitrogen. Thedesired amounts of stabilizers are added to the diisocyanate in three ofthe receivers under an atmosphere of nitrogen, while thetoluene-2,4-diisocyanate in the fourth receiver is used as a control.

Each of the four samples is then divided, again under nitrogenatmosphere, into three portions in screw cap bottles. The cap of thefirst bottle is screwed on tightly and taped and stored over 96%sulfuric acid. The second and third bottles are stoppered loosely sothat the ambient atmosphere can come in contact with the sample, thesecond being stored at room temperature at 20% relative humidity whilethe third is allowed to stand in ordinary room atmosphere. Theunstabilized toluene-2,4- diisocyanate bottles in both the 20% relativehumidity and at room conditions turn light yellow in a few hours and atthe end of 100 hours exposure are brown colored. The stabilized samplesin both the 20% relative humidity and in ordinary atmosphere remainedcolorless at the end of 100 hours exposure. The amount of stabilizingagent is based on the percent by weight of the isocyanate to bestabilized.

Stabilizer Concentration in percent Di-o-tolylcarbonate.Dl-m-tolylcarbonate. Di-p-tolylcarbonate. Guaiacolearbonate(di-o-methoxyphenylcarbonate). Diethylcarbonate. Di-n-amylcarbonate.Dioctadecylcarbonate.

The stabilizing effect on solid isocyanates is shown when thestabilizing agents of this invention are added to them in molten form,for example, when one of the stabilizing agents is added to pure2,4,4-triisocyanatodiphenyl ether, the mixture solidified and broken upunder nitrogen, and then exposed under the same conditions as thosegiven in the example.

Mixtures of isocyanates may of course be stabilized in the same manneras the individual isocyanates.

Any of the aromatic, aliphatic, cycloaliphatic, mono-, di-, or higherpoly-isocyanates which tend to discolor due to the presence of moistureor oxygen, may be effectively stabilized against discoloration by theaddition of the organic carbonates above described. As illustrative ofthe various classes of diisocyanates which may be stabilized againstdiscoloration the following list is given, any of which when substitutedfor the toluene-2,4-diisocyanate of the example, are stabilized againstdiscoloration when tested according to the procedure described therein:

Toluene-2,6-diisocyanate 4,4-methylenediphenylisocyanate4,4-methylene-di-ortho-tolylisocyanate 2,4,4-triisocyanatodiphenyletherToluene-2,4,6-triisocyanate 1-methoxy-2,4,6-benzenetriisocyanateMeta-phenylenediisocyanate 4-chloro-meta-phenylenediisocyanate4,4'-biphenyldiisocyanate 1,5-naphthalenediisocyanate1,4-tetramethylenediisocyanate I,6-hexamethylenediisocyanate1,10-decamethylenediisocyanate 1,4-cyclohexanediisocyanate4,4'-methy1ene-bis(cyclohexylisocyanate)1,S-tetrahydronaphthalenediisocyanate Ortho-, meta-, orpara-tolueneisocyanate Alphaand beta-naphthyleneisocyanate4-methoxy-meta-phenylenediisocyanate The conditions given in the exampleunder which the above stabilizers are tested are quite severe when it isconsidered that compounds such as organic isocyanates are ordinarilyshipped in sealed containers out of contact wtih atmospheric moistureand oxygen, and often light. When such products are stabilized with thecompounds more particularly described above and sealed in iron drums,the effectiveness of the stabilizer will be many times that given fortests carried out in the presence of moisture and air.

I claim:

1. An organic isocyanate of the class consisting of aliphatic,cycloaliphatic and aromatic isocyanates stabilized against discolorationby having dissolved therein from 0.01% to 0.5% by weight, based on theisocyanate,

of an essentially colorless compound of the group consisting of alkyland aryl esters of carbonic acid, the aryl groups in the aryl estersbeing of the benzene series.

2. An organic isocyanate of the class consisting of aliphatic,cycloaliphatic and aromatic isocyanates stabilized against discolorationby having dissolved therein from 0.01% to 0.5% by'weight, based on theisocyanate, of diethylcarbonate.

3. An organic isocyanate of the class consisting of aliphatic,cycloaliphatic and aromatic isocyanates stabilized against discolorationby having dissolved therein from 0.01% to 0.5% by Weight, based on theisocyanate, of di-o-tolylcarbonate.

4. An organic isocyanate of the class consisting of aliphatic,cycloaliphatic and aromatic isocyanates stabilized against discolorationby having dissolved therein from 0.01% to 0.5 by weight, based on theisocyanate, of di-p-tolylcarbonate.

References Cited in the file of this patent UNITED STATES PATENTS2,437,867 Verbanc Mar. 16, 1948 2,476,779 Sturgis July 19, 19492,563,609 Matuszak Aug. 7, 1951

1. AN ORGANIC ISOCYANATE OF THE CLASS CONSISTING OF ALIPHATIC,CYCLOALIPHATIC AND AROMATIC ISOCYANATES STABILIZED AGAINST DISCOLORATIONBY HAVING DISSOLVED THEREIN FROM 0.01% TO 0.5% BY WEIGHT, BASED ON THEISOCYANATE, OF AN ESSENTIALLY COLORLESS COMPOUND OF THE GROUP CONSISTINGOF ALKYL AND ARYL ESTERS OF CARBONIC ACID, THE ARYL GROUPS IN THE ARYLESTERS BEING OF TH EBENZENE SERIES.